Common or ordinary voice grade telephone lines have been utilized for a number of years in connection with the transmission and reception of signals, other than audio signals. Common or ordinary voice grade telephone lines are defined as telephone lines that have substantially the same predetermined or standard bandwidth, i.e. about 300-3400 Hz, and comprise the substantial majority of telephone lines in the United States, as well as in foreign countries, for providing the telephone linkage among residences, public telephones and most businesses. By way of example, common telephone lines, having limited bandwidth, have been used for providing communication between systems or units, such as computers, which are remotely located from each other. Information or data from one computer can be transmitted to and utilized by another computer. Typically, appropriate interfacing between the computers for sending the information or data over the telephone lines is provided by means of a modem.
Ordinary telephone lines have also been used to transmit video signals. The ordinary telephone line, having a bandwidth of about 300-3400 Hz or a transmission rate of about 9.6 kbaud, does not transmit in real time a typical full motion commercial television type digital black and white and/or color digitized video image. The commercial television system displays 512.times.512 pixel images at 30 frames per second and uses about 6 MHz bandwidth when simultaneously transmitting video and audio signals. Because of the large bandwidth required, prior art systems do not enable one to transmit full motion images over an ordinary voice grade telephone line. In connection with the transmission of video signals, it is also required to transmit audio signals. In accordance with one technique for transmitting video and audio signals, the video signal is transmitted over the ordinary telephone line using a first, predetermined bandwidth of the limited bandwidth of the ordinary telephone line and the audio signal is transmitted using a second, predetermined bandwidth of the limited bandwidth of the ordinary telephone line. With respect to this first method, U.S. Pat. No. 4,849,811 to Kleinerman, issued Jul. 18, 1989, and entitled "Simultaneous Audio and Video Transmission with Restricted Bandwidth" describes a system in which modulated digitized image signals and filtered voice signals are transmitted together over an ordinary telephone line whereby still or freeze-frame images are provided with accompanying video. The telephone line has a limited bandwidth, for example, about 300-3500 Hz. The digitized image signals are in the range of 2400 to less than about 4000 Hz. The low pass filter limits the voice signals to a range outside the digitized image signals so that the image signals and voice signals can be transmitted at the same time but over different bandwidths of the limited bandwidth of the telephone line. Because of the separate frequencies, means must be provided for synchronizing the sending and/or receiving of the video and audio signals. In conjunction with the more rapid transmission of video images, the use of known data compression techniques is mentioned in this patent. Similarly, in U.S. Pat. No. 3,873,771 to Kleinerman, issued Mar. 25, 1975, and entitled "Simultaneous Transmission of a Video and an Audio Signal Through an Ordinary Telephone Transmission Line," a communication system is disclosed for transmitting video and audio information using different bandwidths of the limited bandwidth of an ordinary telephone line. With regard to the transmission of video information, it is accomplished using slow scan TV techniques so that an image is not transmitted in real time, but rather the transmission requires up to about 8 seconds to transmit an image with 120 scan lines per image.
In accordance with another technique for transmitting video and audio signals, two signals are multiplexed in such a way to enable one of the two signals to be sent when the other of the two signals is not being transmitted. With respect to this second method, U.S. Pat. No. 4,485,400 to Lemelson, issued Nov. 27, 1984, and entitled "Video Telephone" describes a system for transmitting video information and audio information over a standard or ordinary telephone line. The system automatically multiplexes audio and video signals. When it is determined that sounds or speech are being inputted, video signal transmission is terminated to allow for uninterrupted voice signal transmission. To identify the presence of the audio signal, a tone signal is provided indicative of audio signal transmission. U.S. Pat. No. 4,715,059 to Cooper-Hart et al., issued Dec. 22, 1987, and entitled "Conversational Video Phone" also discloses the separate transmission of audio and video signals. Video image data is transmitted during normal pauses in the telephone conversation. The objective is to permit the transmission of an image frame in less than about 3 seconds. Similarly, U.S. Pat. No. 4,099,202 to Cavannaugh, issued Jul. 4, 1978, and entitled "Multiplexed Communication of Voice Signals and Slow Scan Television Signals Over a Common Communication Channel" describes a system for multiplexing an audio signal with a slow scan television signal. The slow scan television signal includes horizontal sync pulses and the sync pulses are used in determining whether or not voice transmission should be inhibited.
All of the foregoing systems are not capable of transmitting, in substantially real time, audio and moving video image data together over an ordinary voice grade telephone line. Such systems require from about 3-60 seconds to transmit a still image. This occurs because voice grade telephone lines typically have a bandwidth of only about 300-3400 Hz. Because of this bandwidth, the amount of data or information that can be transmitted in a given time is limited. To overcome this drawback, it is known to use transmission lines, other than ordinary telephone lines, for transmitting voice and video data, or some other combination of at least two different sets of data. In such systems, transmission lines having a significantly greater bandwidth than that of ordinary telephone lines, such as fiber optic lines, are utilized. With regard to fiber optic transmission lines or other transmission lines having a much greater bandwidth than the ordinary telephone line, it is known to transmit video and audio signals in substantially real time. U.S. Pat. No. 4,544,950 to Tu, issued Oct. 1, 1985, and entitled "Technique for the Transmission of Video and Audio Signals Over a Digital Transmission Signal" discloses, in one embodiment, a conversion of a standard color video signal and two audio signals to a determined magnitude of Mbit/s optical signal, which is compatible with a predetermined signal format for transmission over a pre-selected light wave line. The system includes a high speed interface multiplexer that combines video information, video mode status information and audio signals into a first signal format. Regarding this resulting signal, two audio bits or two video mode status bits are inserted for every 48 video bits. The simultaneous transmission of two different signals is also disclosed in U.S. Pat. No. 4,237,484 to Brown et al., issued Dec. 2, 1980, and entitled "Technique for Transmitting Digital Data Together with a Video Signal." In accordance with this technique, an inputted video signal is used with a predicted signal to generate an error signal. The error signal is compressed and combined with a supplementary data signal in an adder for subsequent transmission. The supplementary data signal is applied to a transform circuit before being sent to the adder. There is no teaching in the patent of sending the signal output by the adder circuit over an ordinary telephone line. Simultaneous transmission of three television signals is disclosed in U.S. Pat. No. 4,593,318 to Eng et al., issued Jun. 3, 1986, and entitled "Technique for the Time Compression Multiplexing of Three Television Signals." In one embodiment of the system, a time compression multiplexing technique enables the transmission of three color television signals through a satellite transponder having a 36 MHz bandwidth in which one field signal and two field differential signals are each time compressed to permit all three signals to be sent in the period of a normal field signal of a standard TV signal. Since there are three TV sources, with each producing stereo audio, six audio signals are also transmitted. The stereo audio from each source is sent along with the video by inserting digital audio in either the vertical or horizontal blanking periods associated with the video.
In addition to providing an increased bandwidth in order to transmit a plurality of signals including video and audio signals, as some of the foregoing patents indicate, data compression techniques are employed so that compressed video information can be transmitted for subsequent expansion at a receiver station, without meaningful loss of transmitted information. In a publication from the Jan. 26, 1984, issue of Electronics entitled "Codec Squeezes Color Teleconferencing Through Digital Phone Lines" of J. Anderson, S. C. Fralick, E. Hamilton, A. G. Tescher and R. D. Widergren of Widcom Inc., Campbell, Calif., pages 113-115, various compression methods are utilized for transmitting video image data over a digital telephone line at a rate of 56 kilobits/s. The system disclosed in this publication is directed to video signal transmission and not video and audio transmission. In particular, the publication addresses compression at ratios of up to 1440:1. To achieve the compression, spectral, spatial and temporal compression techniques are employed. These data compression techniques are utilized in such a way to exploit the human eye's forgiving nature so as to make the tradeoffs that cause the least objectionable losses in picture quality. In connection with the compression, comparisons are made between new pixel information and previously transmitted pixel information so that only video information that is changing need be sent. The disclosed technique also employs an encoding method that is based on the two-dimensional cosine transform. The use of a state machine is also disclosed for looking up actual codes in Huffman code tables. Although image motion can be above a determined average where more updating is required, typically, only 10% of the pixel information needs to be replenished at the rate of 10 frames/s. The compressed video information is decoded at the receiver so that a resulting 30 frames/s rate of video information can be displayed.
With respect to compression of audio data, in addition to fast cosine transform techniques, it is well known to utilize linear predictive coding (LPC) to reduce or compress audio data being sent over a transmitting medium. Briefly, the predicting of audio data using LPC's is based on an analysis of actual, sampled audio information. Using the sampled audio, mathematical techniques are employed to obtain information that models the audio data. Such information is transmitted. At the receiving end, such audio related information permits an accurate reconstruction of the actual audio. Like the fast cosine transform, LPC techniques permit the use of limited bandwidth transmitting lines, while permitting accurate reconstruction of the actual audio. LPC is discussed, for example, in an article published in Vol. 38, No. 9, September 1990 of IEEE entitled Design and Performance of an Analysis-by-Synthesis Class of Predictive Speech Coders by Richard Rose and Thomas P. Barnwell III.
In sum, many systems have been proposed or devised for transmitting video information and/or audio information, but none has been provided that relatively inexpensively sends and receives, in substantially real time, both video information and audio information over an ordinary voice grade telephone line. It would be advantageous to have such a system in order to provide real time viewing at transmitting and receiving telephones. By doing so, desirable face-to-face contact would be achieved to enhance personal, as well as business, communications. Furthermore, substantially real time viewing of documents and things would result, without meaningful sacrifice of image quality and detail.